At present, a series hybrid vehicle in which a motor is driven by the power generation output of an electric generator driven by an engine and also driven by the discharge output of a battery and the wheels thereof are driven by the motor; a parallel hybrid vehicle in which the wheels are driven by the mechanical output of an engine; and a series-parallel hybrid vehicle capable of running in both a series hybrid mode and a parallel hybrid mode serving as the running modes of these vehicles are known as hybrid vehicles.
In the series hybrid vehicle, the wheels are driven by the electric motor; however, in the parallel hybrid vehicle, although the wheels are driven by the mechanical output of the engine, at the time of, for example, starting, acceleration and braking, the wheels can be driven so that the difference between the mechanical output of the engine and a requested output is made up for by using a rotating machine provided on the axis of the engine. In this case, acceleration is attained by making the rotating machine to act as an electric motor and deceleration is attained by making it to act as an electric generator; at the time, the battery mounted on the vehicle supplies electric power to the rotating machine (electric motor) or electric power is regenerated from the rotating machine (electric generator).
The capacity (SOC) of the battery mounted on the vehicle is herein charged not only by the regenerative electric power of the rotating machine and the electric power from an external power source but also by the power generation output of the electric generator driven by the engine. In particular, when the capacity of the battery becomes lower than a predetermined capacity while the vehicle is running, charge control is repeated by the electric power of the electric generator driven by the engine, whereby the battery capacity (SOC) is suppressed from becoming excessively low.
In the case of the series-parallel hybrid vehicle, a mechanism, such as a clutch, is used so that a mechanical connection is made between the electric generator and the electric power so as to be disengageable and engageable. When the series-parallel hybrid vehicle is run as a series hybrid vehicle, the clutch is disengaged so that the mechanical connection between the electric generator and the electric motor is separated. As a result, the power generation output of the electric generator driven by the engine is supplied to the electric motor via the battery. In this state, the vehicle can run as a series hybrid vehicle. Conversely, in the case that the vehicle is run as a parallel hybrid vehicle, the clutch is connected so that the mechanical connection is made between the electric generator and the electric motor. As a result, a state is obtained in which the mechanical output of the engine is mechanically transmitted to the drive wheels via the electric generator, the clutch, and the electric motor, and in which the vehicle can be accelerated or decelerated using the electric generator and the electric motor. In this state, the vehicle can run as a parallel hybrid vehicle.
An example of this kind of series-parallel hybrid vehicle is disclosed in Patent Document 1 (JP-A-2000-209706). In this series-parallel hybrid vehicle, two motors capable of being driven as an electric generator and an electric motor by switching are provided between the output shaft of the engine and the drive shaft on the side of the drive wheels, and a clutch and a brake are provided therebetween. When the vehicle runs in the series hybrid mode, the clutch is disengaged, the motor on the front side is made to act as an electric generator, and the motor on the rear side is made to act as an electric motor; when the vehicle runs in the parallel hybrid mode, the clutch is engaged and the vehicle runs on the rotation of the engine, or the motors on the front and rear sides are driven to make acceleration/deceleration easy.
The operation range of the motors mounted on the series hybrid vehicle is divided into a series hybrid mode range (including an EV mode range in which the engine is stopped and the motors are driven) and a parallel hybrid mode range, and the switching between the two modes is performed by the switching of the clutch that is controlled by controlling means.
For example, in Patent Document 1, controlling means performs mode switching control by using a motor operation range setting map. Various kinds of information, such as vehicle speed obtained by a vehicle speed sensor, requested torque calculated on the basis of the position of the accelerator pedal and the vehicle speed, and the remaining capacity of the battery obtained from a remaining capacity sensor, are herein input to the controlling means. Then, the controlling means basically performs control to select the series mode when the engine is in an operation state in which the engine is started/stopped, to start the engine and thereby to generate electric power using the electric generator so as to charge the battery in the case that the remaining capacity of the battery becomes equal to or less than a predetermined value, and to stop the operation of the engine and thereby to stop the power generation at the electric generator so as to prevent the battery from being overcharged in the case that the remaining capacity of the battery becomes equal to or more than the predetermined value. Furthermore, a range in which the requested torque is low is set as the series hybrid mode range, and a range in which the requested torque is high is set as the parallel hybrid mode range. The controlling means determines the running mode range, then performs clutch switching control so that the present running mode range becomes the determined target running mode range or performs switching control via inverters so that the functions of the motors are used as the functions of the electric generator and the electric motor.
In addition, a series-parallel combined hybrid vehicle system having a drive system in which a continuously variable transmission and a clutch are connected in series between an electric generator on the side of the engine and a motor on the side of the drive wheels is disclosed in Patent Document 2 (JP-A-4-297330). A series running mode in which the vehicle runs using the motor is herein usually used; in a power shortage range, the clutch is connected and a parallel running mode is selected, and then the transmission gear ratio of the continuously variable transmission is controlled to drive the engine in an operation range in which fuel efficiency is high. Furthermore, the regeneration torque in a high-speed rotation range at the time of regenerative braking is absorbed by engine friction by controlling the transmission gear ratio.